As an optical space transmission apparatus for performing optical telecommunication between two points separated remotely, there is known a parallel 2-axis type one in which an optical transmission system and an optical reception system are provided respectively and mirror cylinders respectively incorporating the optical transmission system and the optical reception system are provided in parallel to each other. This apparatus becomes large in size, and requires adjusting means for adjusting parallelism between both of the mirror cylinders so that the apparatus becomes complicated in structure.
Accordingly, if the optical transmission system and the optical reception system are formed as a coaxial type, the above-mentioned problems can be solved.
A comparative example of an optical space transmission apparatus of coaxial type will be described next with reference to FIG. 1. Reference numeral 1 designates a mirror cylinder which incorportates therein an optical information transmission light emitting element 5, an optical information reception light receiving element 9 and a half mirror 10 which separates transmission and reception lights. Also, a transmission/reception common lens 3 is mounted to the opening of the mirror cylinder 1.
A diverging transmission light L1 from the light emitting element 5 is introduced through the half mirror 10 to the transmission/reception common lens 3. The substantially parallel transmission light L1 therefrom is transmitted to an optical space transmission line 8, then transmitted thereform and received by an optical space transmission apparatus at the reception side.
A substantially parallel reception light L2 transmitted from the apparatus at the reception side through the optical space transmission line 8 is introduced into the transmission/reception common lens 3. A converged reception light L2 therefrom is reflected by the half mirror 10 and then introduced into the light receiving element 9. The reception light L2 incident on the light receiving element 9 is therein converted into an electrical signal.
In the optical space transmission apparatus shown in FIG. 1, however, the transmission light L1 and the reception light L2 are both lost by the insertion loss of the half mirror 10 and S/N ratio of the transmission light L1 and the reception light L2 is lowered by a stray light produced by the half mirror 10 within the mirror cylinder 1, thus making it impossible to perform the long-distance communication.
In view of the above aspect, the present invention is to provide an optical space transmission apparatus of simple construction which can improve transmission efficiency and S/N ratio of both the transmission and reception lights.